A DEFINITION OF EVOLUTION 



classes of vertebrates. Arch three serves the head region, arch four (but 

 only one of the pair) serves the systemic circulation, and arch six serves 

 the lungs. Yet all six pairs are developed in the embrvos of birds and 

 mammals. 



A similar story could be told with respect to almost any organ system 

 in any major group. The details differ, but the general facts are the same. 

 When, in the course of embryonic development, a new organ system is 

 formed, its structure is closely similar even in the most widely dissimilar 

 species of the same class, or even phylum in many instances. As differen- 

 tiation proceeds, detectable differences first become apparent between the 

 embryos of those species of which the adults are most widely different. 

 Thus the very early embryos of fishes and of mammals are quite similar, 

 but they soon become recognizably differentiated. As development pro- 

 ceeds, the similarities of the embryos become progressively restricted to 

 smaller and smaller taxonomic groups until finally the characters which 

 distinguish the adults of closely related species are formed. In many 

 instances, this process is not completed until after birth (or hatching). 

 Thus Juvenal robins have the speckled breast which is typical of the adults 

 of most species of the thrush family, to which the robin belongs. 



Many examples indicate relationship within as well as between classes. 

 Thus the whalebone whales, which feed by straining minute organisms 

 out of the sea water, have no teeth in the adult stage, yet their embryos 

 have a set of tooth buds which are resorbed without ever erupting. Whale 

 embryos also have a coat of hair which is entirely lost to the adults. The 

 absence of birds' teeth is proverbial, yet their embryos also have ephem- 

 eral tooth buds. Such facts are readily understandable on the principle 

 that the basic factors of embryology are determined by heredit\', and so 

 are common to related species and groups. Both whales and birds are 

 descended from ancestors which had teeth. The hereditary factors which 

 initiate tooth development are still present and active. But an additional 

 hereditary change (mutation) which acts later in development has been 

 independently acquired in each group since its origin. And this change 

 causes the tooth buds to abort. 



Much more radical changes in developmental pattern may also occur, 

 so that only the early embryonic stages can give a clew to the true rela- 

 tionships of the organisms involved. Some extreme examples occur among 

 the cirripedes (barnacles). Typical cirripedes show many typical crusta- 

 cean characters, so that it is not difficult to understand that thev belong in 

 this subphylum. Nonetheless, they are sufficiently aberrant as to have 

 deceived so competent a zoologist as Cuvier, who treated them as a class 

 of the Mollusca. Yet the larvae of all cirripedes are immistakably crusta- 

 cean larvae, and it was this fact which finallv established the correct 

 taxonomic position of the barnacles. A much more extreme example is 

 presented by Sacciilina, a very aberrant barnacle which parasitizes crabs. 

 This organism goes through all of the tvpical developmental stages of a 

 barnacle vmtil it settles down upon the abdomen of its host. At this point, 

 other barnacles would develop the usual adult structures of the group, 

 but Sdcculinn undergoes a degenerative development. Appendages are 



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